The current studies are designed to follow these earlier observations, and to investigate genomic instability as a function of neutron energy. Apart from the mechanistic implications of these investigations, studies of slow-neutron induced genomic instability are relevant because of the significant population of individuals who are occupationally exposed to low-energy neutrons. These studies will attempt to test whether low-energy neutron irradiation is capable of inducing genetic instability in human mammary epithelial cells. To date studies of radiation induced genomic instability have been limited and largely phenomenological and thus data from these reports have provided few insights into its nature or potential mechanisms. In this regard, potential differences as a function of LET, have been suggested which have important mechanistic implications which may relate to reported differences in the nature of radiation-induced mutagenesis and carcinogenesis. Further, specifically with regard to mammary carcinogenesis there are indications of qualitative as well as quantitative differences in the effectiveness of neutrons relative to photons. Clearly, before the contribution of genomic instability to radiation induced cancer can be ascertained, a more complete understanding of this phenomenon is necessary. Toward this end, the present studies will investigate genetic instability at the chromosomal and DNA levels, in human mammary epithelial cells exposed to photons relative to low energy neutrons.